There is increasing commercial interest in integrating fiber optical systems with photonic integrated circuits (PICs; also called “photonic chips”) to create photonic-integrated-circuit systems. Such systems can be used to form a variety of devices, such as antenna systems (e.g., phased-array antennas), interferometers (e.g., Mach-Zehnder interferometers), biosensors and spectrometers.
PIC systems typically require mechanisms for coupling light into and/or out of optical waveguides of the PIC with a reasonable coupling efficiency. Single-mode fibers (SMFs) are conventionally used for grating-based, fiber-to-chip optical coupling. The conventional wisdom is that SMFs with a small mode-field diameter (MFD) (e.g., less than the conventional MFD of about 10 microns (μm) for conventional SMFs) would have a reduced coupling efficiency between a given fiber and the corresponding on-chip waveguide due to the smaller number of grating periods overlapping with the mode field. This has resulted in a lower limit on the footprint (area) of the grating couplers in silicon-on-insulator (SOI) integrated photonic systems. Yet, it would be advantageous to have as small a footprint as possible to increase the density of the optical connections for photonic-integrated-circuit systems.